Adenosine is a potent modulator of synaptic transmission in the central nervous system, with the capability of inhibiting responses by >95% at some synapses. However, many aspects of its functional role remain to be determined. The present experiments will address a number of related issues aimed at defining this role, using electrophysiological techniques to study the effects of adenosine on evoked excitatory synaptic responses in brain slice preparations. The first set of experiments will investigate the cellular mechanism(s) by which adenosine blocks the release of neurotransmitter at 3 excitatory amino acidergic synapses, namely the Schaffer collateral/commissural input to the CA1 region of the hippocampus, the lateral olfactory tract input to the olfactory cortex, and the cortical input to the striatum. The effects of phorbol ester, lithium, and potassium channel blockers on responses to adenosine will be characterized. The second set of experiments will use patch electrode recording in the whole-cell configuration in combination with a statistical analysis of the variance in small EPSPs to better define the effects and site of action of adenosine, other neuromodulators, and antagonists at hippocampal synapses. The third set of experiments will focus on the postsynaptic actions of adenosine. The effects of the A2a receptor-selective agonist CGS 21680 will be characterized in striatum, the "A1-like" receptors (possibly A3?) that mediate the postsynaptic effects of adenosine in the hippocampus will be studied with receptor subtype selective drugs, and the relationship between K+ currents activated by adenosine and channels regulated by intracellular ATP (K+ [ATP] channels) will be determined. Finally, electrophysiological and pharmacological techniques will be employed to quantitatively determine the sensitivity of hippocampal slices to adenosine, and investigate the factors that regulate the extracellular concentration of adenosine in the slice. The studies detailed in this proposal should improve our understanding of the receptors upon which adenosine acts, the cellular mechanisms that mediate adenosine responses, and the role played by synaptic modulation in normal brain activity. Although adenosine receptor antagonists such as caffeine and theophylline have some therapeutic actions, there are currently no clinical uses for adenosine receptor agonists. The basic information that will be learned from the proposed experiments about the role of adenosine in the nervous system might suggest possible clinical applications for these drugs.